Boron carbide bonding

Mechanical properties Symbol Units Silicon carbide, sintered Silicon carbide, silicon- infiltrated Silicon carbide, recrystallized Silicon carbide, nitride-bonded Boron carbide... 

The mechanical properties of self-bonded boron carbides produced by different densification techniques are summarized in Table 11.2. 

All the values might be slightly elevated relative to traditional reaction bonded SiC (RB-SiC as shown on the plot ) because of the reduction in residual silicon due to the copper additions. It has been found that copper additions reduce the residual silicon content and may form copper silicides. This was observed in a hybrid SiC/B4C composite with copper and iron additions to the infiltration alloy. The XRD data from that study suggested that as the metallic additions were increased, the silicon content decreased and it has been shown that the hardness increases with decreasing silicon content in reaction bonded boron carbide materials. 

M, Dariel and N. Frage, Reaction bonded boron carbide recent developments, Adv. AppL Ceram., Ill, [5-6], 301-310, (2012). 

P. Karandikar, S. Wong, G. Evans, and M. Aghajanian, Miaostruaural development and phase changes in reaction bonded boron carbide, CESP, 31, [5], 251-259-167, (2010). 

In general, the purified boron carbide is ultimately obtained as a granular soHd that subsequendy may be molded or bonded into usehil shapes. To achieve high density and strength, it is hot pressed at 1800—2400°C in graphite molds. 

Boron carbide is used in the shielding and control of nuclear reactors (qv) because of its neutron absorptivity, chemical inertness, and radiation stabihty. For this appHcation it may be molded, bonded, or the granular material may be packed by vibration. 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

Properties. Boron carbide has a rhombohedral structure consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms in a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be B12C3 [12075-36 4], Each boron atom is bonded to five others in the icosahedron as well as either to a carbon atom or to a boron atom in an adjacent icosahedron. The structure is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B12C3 is 2.52 g/mL. The rigid framework of... 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

Elemental boron is a refractory material that is usually isolated either as a shiny black crystalline solid or a softer, browner, more impure amorphous solid. Reduction of readily available boron compounds containing boron oxygen bonds to elemental boron is energy intensive and costly. This has limited the extent of conunercial use of this material. Many related refractory boron compounds have been prepared and characterized including metal borides, boron carbides, boron nitrides, and various boron metal alloys. These refractory materials and elemental boron are also discussed in some detail in the article Borides Solid-state Chemistry. Other general references are available on elemental boron and other refractory boron compounds. " ... 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

The B mas and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick etal. 1991). A small additional shoulder on the major resonance of the static B spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

Compare oxide ceramics such as alumina (AI2O3) and magnesia (MgO), which have significant ionic character with covalently bonded nonoxide ceramics such as silicon carbide (SiC) and boron carbide (B4C see Problems 19 and 20) with respect to thermodynamic stability at ordinary conditions. 

Figure 3. Interaction between boron carbide and type 316 stainless steel bonded by sodium. (After Ref. 9.)...

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